The bass clarinet performs bass tasks in the orchestra and is combined with all the other bass instruments to fulfill this role. The characteristics of the low notes mean they are also used to play eloquent, solemn and emotive solos at moderate tempos. The full-sounding upper register is very well suited to thematic tasks (middle voices).
Bass clarinet + other woodwind instruments
Bass clarinet + clarinet sound very homogeneous when played together, either in chords or with the bass clarinet an octave lower.
The bass clarinet also combines well with the deep woodwinds, for instance with the English horn in unison or an octave above. In combination with the bassoon in octaves or chords the bass clarinet usually provides the foundation, to avoid its weak middle register. The bass clarinet in combination with the contrabassoon playing an octave lower sounds very full.
Bass clarinet + brass
Of the brass, the French horn is especially well suited for combinations with the bass clarinet; both instruments have a substantial and mellow sound, a high degree of blending is possible.
Bass clarinet + strings
Bass clarinet and cellos in unison is a combination found in a great many scores which produces a particularly soft overall sound.
The bass clarinet in its low register can also play in unison with the double-basses. The sound is then very dark and rumbling.
Full-sounding combinations, especially at piano levels, are also possible with the harp.